1. Field of the Invention
The present invention relates generally to hydraulic control valves, and more specifically to a pulsating hydraulic valve for causing and controlling reciprocal action of a hydraulic cylinder.
2. State of the Prior Art
There are many applications for hydraulic cylinders wherein continuous reciprocating motion is required. Essentially, a hydraulic cylinder is a common generic term for a linear hydraulically driven actuator comprising a cylindrical housing with a piston positioned slideably in the housing. A piston rod attached to the piston extends through appropriate seals out one end, the rod end, of the cylindrical housing, and the opposite or blind end of the cylindrical housing is usually enclosed, although some hydraulic cylinders have rods extending out both ends of the cylindrical housing. Hydraulic fluid inlet and outlet ports are positioned on opposite sides of the piston for admitting hydraulic fluid under pressure into the cylinder and allowing the fluid to escape the cylinder. Of course, pressurized hydraulic fluid flowing into the cylinder on one side of the piston forces the piston to move in the opposite direction, and the piston rod can be connected to any apparatus desired to be moved. Such hydraulic cylinders with fluid ports positioned at opposite ends, i.e., on both sides of the piston, as described above, are commonly called double acting cylinders. Reciprocal motion of the piston rod in a double acting cylinder is caused by alternately directing pressurized hydraulic fluid into the cylinder on one side of the piston and then on the other side.
Common hydraulic control valves have spools with annular cavities in their peripheral surfaces positioned slideably inside bores in valve housings. The spool shuttles back and forth to open and close selected ports in the valve housing. For example, in one spool position, a cavity in the spool can connect a port delivering pressurized hydraulic fluid with a port directed to one end of a double acting cylinder while simultaneously connecting a port from the opposite end of the cylinder with a drain or conduit to a hydraulic fluid reservoir or tank. Thus, pressurized hydraulic fluid is directed by the control valve to one side of the piston causing movement of the piston and rod in the one direction while allowing hydraulic fluid from the opposite side of the piston to escape by draining to the tank. Alternately, shifting or shuttling the spool to a different position could connect the opposite cylinder ports to high pressure fluid and tank, respectively, to cause the piston and rod to move in the opposite direction.
Shifting or shuttling the spool from one position to the other, as described above, usually just involve moving it longitudinally within the valve bore. Such shuttling can be actuated or accomplished by a hand-operated lever connected to the spool, a solenoid actuator, or even hydraulic actuators that apply hydraulic fluid pressure to one end or the other of the spool.
The problem with those conventional shuttle valve control devices when reciprocating motion is required, especially over extended periods of time, is that they require some kind of external control devices. A solenoid-actuated valve requires electric switches, either position-actuated by sensing the physical position of a part or pressure actuated by sensing hydraulic pressure in various portions of the hydraulic system. Fluid actuated valves also require some kind of equipment position sensors and external valving apparatus. Over extended periods of time, these kinds of external shuttle control actuators or control devices tend to wear out, break down, or otherwise become unreliable or require maintenance. They also add significantly to the manufacturing costs of reciprocating hydraulic devices.